Decarburization in casting of steel



Uflitd States Pat n METHOD OF CONTRGLLING COOLING AND DECARBURIZATION IN CASTING OF STEEL 1N SHELL MULDS Theodore Operhzill, Whitehall, and Charles Yaker, North Muskegon, Mich, assignors to Howe Sound Company, New York, N.Y., 'a corporation of Delaware N0 Drawing. Filed Apr. 7, 1961, Ser. No. 101,339 6 Claims. (Cl. 22 -215) This invention relates to the production of castings by ceramic metal casting processes and it relates more particularly to a method of improving the surface quality of castings and decreasing oreliminating the decarburization prevalent in some castings.

Present techniques for providing ceramic shell metal casting molds generally include the steps of building up a plurality of stucco and dip coats onto the surface of a pattern of material which is disposable at elevated temperatures. The pattern material is subsequently disposed of by firing and the ceramic coatings are matured; the molds then being of a character adaptable for introduction of the molten metal.

The molds which are provided by the above noted techniques have many advantages. The molds are porous, but at the same time have low permeability and they provide excellent heat conductivity characteristics.

In spite of these advantages, the use of shell molds has not completely solved a problem long recognized in investment casting. This problem which is particularly predominant in alloy steels, relates to the tendency of the metal to pit and/or decarburize during pouring and during solidification. Shell molding has eliminated the bulky type molds which were employed in conventional investment casting processes. However, they have not completely eliminated the tendency for decarburzation and pitting, for example, when casting such metals as 410 stainless steel.

It is therefore an object of this invention to provide an improved technique for the production of castings by ceramic shell mold metal casting processes.

It is an additional object of this invention to provide a ceramic shell mold metal casting technique which substantially eliminates surface defects and decarburization in the castings produced. I

It is a further object of this invention to provide a ceramic shell mold metal casting technique which enables the production of castings without surface defects or decarburization and which at the same time provides a means for improving the structure and heat treating characteristics of the castings.

These and other objects of this invention will appear hereinafter and it will be understood that the specific embodiments hereinafter set forth are presented for purposes of illustration only.

The present invention generally relates to the handling of the ceramic shell molds before and during the pouring operation and the processing immediately thereafter. The process consists of providing a mass of carbon containing material, preferably in particulate fonn, about the mold or cluster of molds. The carbon containing material has been found to provide conditions which effectively prevent decarburization and pitting. In addition, the material has been found to improve the grain structure at the surface of the casting treated in accordance with this method. The closely associated mass of carbon containing material which is provided at room temperature about the molds permits rapid cooling of the castings. This rapid cooling avoids grain growth at the surface of the castings and provides more uniform mechanical properties and response to heat treatment of the castings.

An example of the procedure employed in carrying out 3,125,597 Patented Mar. 31

the process of this invention will serve to illustrate the features thereof. In accordance with the practice set forth in the application of Theodore Operhall et al., Serial No. 708,628, filed January 13, 1958, and now Patent No. 2,961,751, a cluster of wax patterns provided by conventional techniques is first given a dip coat of the'following composition:

8000 cc. colloidal silica:

(30% grade) (Specific gravity, 1.198) 165 pounds zircon:

(99% through 325 mesh) (65-67% ZnO 34-32% SiO 6150 cc. water 110 grams sodium fluoride After dip coating, a first stucco layer of Alundum (109% through 50 mesh with less than 3% through 100 mesh-better than 90% between 60 and 80 mesh) is provided. This procedure is followed by alternate layers of dip coat and stucco coats, the number generally varying between about 5 to 10 coats or more.

Following the formation of the shell about the cluster, a firing operation commences which results in dewaxing. After the firing stage, the cluster may be preheated in accordance with conventional casting practice and thus prepared to the pouring operation. During pouring, the self-supporting mold may be clamped to a furnace and poured as shown in FIGURE 5 of the aforementioned patent. Immediately after pouring, the mold will be transferred to a flask or the like wherein a surrounding mass of the carbon containing material is provided. The mold could be lowered into a loose mass of this material, or the material can be introduced by flowing around'the mold after insertion into the flask or other chamber. t

Alternatively, the pouring into the molds may take place while the molds are surrounded by the carbon containing material. In this embodiment, the molds would be introduced into the carbon containing material which is contained in the flask or the material may be flowed about the molds in the flask. After this, the molten metal would be poured. If the latter embodiment is resorted to, his necessary to pour immediately after surrounding the molds with the carbon containing'material to avoid defeating the purpose of preheating and to utilize the maximum chilling effect of the material.

In the case of relatively large castings, it is sometimes necessary to provide refractory substances for supporting the matured molds during the preheating and pouring stages. The concepts of this invention may be suitably accomplished with such large castings if the refractory supporting material is replaced by or mixed with the carbon containing material. The molds employed for these castings can thus be provided in a flask or like chamber prior to preheating, surrounded by the carbon containing material, or a mixture thereof with the refractory material, and then preheated, poured and cooled while thus supported. The refractory material employed in the back-up may be any well known refractory substance such as alumina, chromite, magnesite or zirconia, and it may be employed in amounts up to about by weight of the total back-up.

As an additional embodiment of this invention, it is contemplated that only portions of the shell mold may be surrounded by the carbon containing material, for example, areas which would be particularly susceptible to cooling and thus would show a greater tendency towards decarburization, pitting and poor grain structure. In addition, it is contemplated that regions of carbon containing material adjacent the shell mold surface be alternated with areas of insulating refractory material adjacent certain other areas on the shell mold outer surface.

3 The latter situation may be necessary where particularly intricate areas of the castings would crack if chilled too rapidly.

Additionally, the carbon containing material may be selectively applied in situations where directional solidification is desired. Hence, the use of the material in locations in which a fast rate of solidification is required will provide an improved thermal gradient and assist materially in the structure and solidity of the cast product.

Suitable carbon containing materials adaptable for use in accordance with this invention include conventional cast iron grit and flake or chip graphite. Various other materials comprised of or containing substantial amounts of carbon and which have relatively good heat conductivity are obviously also adaptable for use in this process.

The grit, graphite and other materials which are employed are preferably provided in sizes of about 10 mesh to insure substantial contact with all portions of the outer surface of the shell. However, the size of the particles is not critical, provided the material can be flowed about the mold and substantially surrounded. The layer of carbon containing material surrounding the shell molds should be at least about 1 inch thick. However, when flowed about a mold or cluster in a flask, a layer at a given point may extend for several inches. This is not a critical factor.

It is apparent from the above that the surrounding material is preferably a solid substance which reduces oxidation and has the ancillary elfect of aiding in the chilling and selective solidification of castings. It is contemplated however, that non-solid substances may be incorporated along with the carbonaceous materials to aid in achieving the advantages of this invention. In this connection, it is conceivable that the shell mold be placed in a chamber containing a non-oxidizing or reducing gas while at the same time surrounded with the solid materials. Argon, nitrogen, and carbonaceous gases are suitable examples of such gases and the amounts thereof can be readily varied in accordance with the size and the oxidizing, decarburizing and pitting tendencies of the cast metal.

It will be understood that various modifications may be made in the above disclosed ceramic shell mold casting process which provide the characteristics of this invention without departing from the spirit of this invention, particularly as defined in the following claims.

We claim:

1. In a metal casting process which includes the steps of building up stucco and dip coats of finely divided ceramic material onto the surface of a pattern which is disposable at elevated temperatures whereby the said l pattern is enclosed within a shell of the ceramic material except for an opening for introduction of casting metal, and firing the assembly to dispose of the pattern and leave a corresponding cavity within the shell and to mature the ceramic materials in the shell, the improvement comprising pouring metal in a molten state through said opening into the cavity to produce a metal casting, surrounding the shell with a mass of solid particulate carbon containing material immediately after pouring of said metal to bring said material into contacting relationship with the outer surfaces of the shell to extract heat rapidly from the metal poured into the shell to chill the surface whereby the surface portions are not subjected to carburization while the poured metal is rapidly solidified to avoid grain growth and permitting the metal to solidify while so surrounded.

2. The process according to claim 1, wherein said carbon containing material comprises cast iron grit.

3. The process according to claim 1, wherein said carbon containing material comprises graphite chips.

4. In a metal casting process which includes the steps of building up stucco and dip coats of finely divided ceramic material onto the surface of a pattern which is disposable at elevated temperatures whereby the said pattern is enclosed within a shell of the ceramic material except for an opening for introduction of casting metal, and firing the assembly to dispose of the pattern and leave a corresponding cavity within the shell and to mature the ceramic materials in the shell, the improvement comprising surrounding the shell with a solid mass of particulate carbon containing material to bring said material in contact with the outer surfaces of the shell to extract heat rapidly from the metal poured into the shell to chill the surface whereby the surface portions of the poured metal are not subjected to carburization while the poured metal is rapidly solidified to avoid grain growth, subsequently pouring molten metal into the shell to produce a metal casting and permitting the metal to solidify while surrounded by said mass.

5. The process according to claim 4, wherein said carbon containing material comprises cast iron grit.

6. The process according to claim 4, wherein said carbon containing material comprises graphite chips.

References Cited in the file of this patent UNITED STATES PATENTS 2,893,084 Eisermann July 7, 1959 2,961,751 Operhall et al Nov. 29, 1960 FOREIGN PATENTS 752,742 Great Britain July 11, 1956 

1. IN A METAL CASTING PROCESS WHICH INCLUDES THE STEPS OF BUILDING UP STUCCO AND DIP COATS OF FINELY DIVIDED CERAMIC MATERIAL ONTO THE SURFACE OF A PATTERN WHICH IS DISPOSABLE AT ELEVATED TEMPERATURES WHEREBY THE SAID PATTERN IS ENCLOSED WITHIN A SHELL OF THE CERAMIC MATERIAL EXCEPT FOR AN OPENING FOR INTRODUCTION OF CASTING METAL, AND FIRING THE ASSEMBLY TO DISPOSE OF THE PATTERN AND LEAVE A CORRESPONDING CAVITY WITHIN THE SHELL AND TO MATURE THE CERAMIC MATERIALS IN THE SHELL, THE IMPROVEMENT COMPRISING POURING METAL IN A MOLTEN STATE THROUGH SAID OPENING INTO THE CAVITY TO PRODUCE A METAL CASTING, SURROUNDING THE SHELL WITH A MASS OF SOLID PARTICULATE CARBON CONTAINING MATERIAL IMMEDIATELY AFTER POURING OF SAID METAL TO BRING SAID MATERIAL INTO CONTACTING RELATIONSHIP WITH THE OUTER SURFACES OF THE SHELL TO EXTRACT HEAT RAPIDLY FROM THE METAL POURED INTO THE SHELL TO CHILL THE SURFACE WHEREBY THE SURFACE PORTIONS ARE NOT SUBJECTED TO CARBURIZATION WHILE THE POURED METAL IS RAPIDLY SOLIDIFIED TO AVOID GRAIN GROWTH AND PERMITTING THE METAL TO SOLIDIFY WHILE SO SURROUNDED. 